Computer printers are efficient, quiet and produce high quality printed images in a relatively inexpensive manner when operated in low speed printing modes. Image quality is achieved by sweeping indicia depositing apparatus over a print medium and depositing indicia forming material onto the medium in desired indicia patterns. The combined effect of sweeping, depositing and sequentially moving the medium through a predetermined print zone in a series of desired steps, enables the formation of a series of swaths that defined a desired image configured in an N by M image matrix array. The quality of the print image is then determined by applying the indicia forming material in a precise manner to the individual pixel locations on the print medium without any substantial coalescence.
While such low speed printers have been satisfactory for many applications, there has been a constant demand for printers to produce higher quality photo-like full color images. Meeting the demand for producing photo quality, high color density images however, has not been achieved easily. In this regard, in order to produce full vibrant colors on a print medium, large volumes of indicia forming material must be deposited in concentrated areas on the medium. While depositing large volumes of indicia forming material on a print medium produces vibrant colors, such large deposits in adjacent pixel locations often results in some form of coalescence or observable artifacts.
One common form of indicia forming material utilized in modern day computer printers, such as inkjet printers, is ink carried in a water based media. The water based ink solution permits the deposit of large volumes of ink in a fast and efficient manner. The quality of the ink patterns formed on the print medium is then highly dependent upon the volume of the individual ink droplets and the accuracy in their placement on the medium pixel locations.
While inkjet printers produce high quality photo-like images, it is well known to those skilled in the art, that droplet size and placement accuracy degrade with time due to mechanical and electrical fatigue factors associated with the print head nozzles of such printers. Such nozzle fatigue factors generally manifest in pattern-like printing errors that are quite noticeable to a user particularly when printing in a single pass printing mode.
In order to help extend the useful life of such print heads, many inkjet printers have employed what is known as a multi-pass print mode of operation that facilitates the hiding of individual nozzle failures. For example as a comparison, in a single pass mode, should a given nozzle not fire perfectly, be misdirected, or clog and fail, the degradation in print quality will be seen in each row of the image printed by the defective nozzle. On the other hand, in a multi-pass print mode of operation each row is printed by two or more nozzles. In this manner, for example, in a four pass print mode printer only one out of four ink droplets may be missing or misdirected resulting in a much less catastrophic result. While multi-pass printing has helped improve image quality, the improvement has been at the expense of throughput.
One attempt at helping to improve throughput while simultaneously addressing the problems associated with coalescence as well as bleeding and beading of ink droplets between adjacent pixel locations, is disclosed in U.S. Pat. No. 4,748,453. In this regard, there is taught an ink jet printing method for depositing drops of ink in a checkerboard pattern through the use of a mask. The use of the mask assures that there is no overlap of ink spots from adjacent pixel areas when the ink is still in a flowable state. While the dividing the ink drop deposits into separate and distinct complementary areas has improved the problems associated with coalescence, such masking techniques have not entirely solved the problems associated with coalescence since ink deposits between diagonally oriented pixel locations still exhibit coalescence. Moreover, the use of small masks, such as 2.times.2 masks and 4.times.4 masks, produce artifacts as the fixed mask configuration is applied in a highly repetitive manner. For example, a 2.times.2 mask is typically applied every two pixels causing a repetitive pattern to be stamped out every two pixels in both the horizontal and vertical directions and especially in the horizontal direction. Such a repetitive pattern becomes visible to a user when the nozzles do not fire perfectly, are misdirected, mis-aligned or clogged. In this regard, as a defective nozzle sweeps across a print medium, the resulting swath will exhibit the same dot absence or misplacement for every pixel produced from the defective nozzle.
Another attempt at solving the problems associated with coalescence between diagonally adjacent pixel locations is disclosed in U.S. Pat. No. 4,965,593 known as the Hickman patent. The Hickman patent teaches separating the ink drop deposits in every direction, in each pass, by leaving one blank pixel in the pixel grid spacings. While this technique solves the coalescence problem associated with diagonally adjacent pixel locations, the resulting inability to print on the spaced apart pixel locations has proven to be less than desirable.
Thus, while the use of a multi-pass print mode in combination with a mask has been successful in some applications, this technique has not been entirely satisfactory. Therefore it would be highly desirable to have a new and improved printer and masking method for depositing ink droplets onto a print medium that substantially eliminates or at least that greatly reduces the depositing of indicia forming material in adjacent pixel locations while simultaneously helping to substantially eliminate or at least greatly reduce unwanted and undesired artifacts caused by the repeated application of a fixed mask matrix array.